[leetcode]Longest Univalue Path

要注意边和节点数是不一样的

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def longestUnivaluePath(self, root: TreeNode) -> int:
        result = [0]
        
        def backtrace(node, result):
            if not node:
                return 0
            leftPathMax = backtrace(node.left, result)
            rightPathMax = backtrace(node.right, result)
            ret = 0
            if node.left and node.val == node.left.val:
                ret = max(ret, leftPathMax + 1)
                result[0] = max(result[0], ret)
            if node.right and node.val == node.right.val:
                ret = max(ret, rightPathMax + 1)
                result[0] = max(result[0], ret)
            if node.left and node.right and node.val == node.left.val and node.val == node.right.val:
                result[0] = max(result[0], leftPathMax + rightPathMax + 2)
            return ret
        
        backtrace(root, result)
        
        return result[0]